1. Field of the Invention
The present invention relates to a surface-wave arrangement which has interdigital transducers and, possibly, further structures on the surface of a piezoelectric substrate. The interdigital transducers are used to convert electrical signals into acoustic surface waves, and vice versa. Further structures can be resonator structures, reflector structures or even conversion structures, as are known from WO 89/02675.
2. Description of the Related Art
Acoustic waves, which are designated by the general term "surface waves" and are Rayleigh waves, Bleustein waves, surface-skimming bulk waves and the like, run on or in the surface or in the region close to the surface of the respective substrate body. Apart from aperture effects and diffraction effects, such acoustic waves run in a direction which is designated as the main wave propagation direction, having wave fronts which are transverse to this direction and are in a straight line corresponding to the transducers, the opposing direction also being included. Specifically, such acoustic waves are transmitted from an interdigital transducer in the direction and the opposing direction as the main wave propagation direction, unless this transducer is not specifically constructed as a unidirectional transducer.
The substrate body of such a surface-wave arrangement is a platelet, which is normally elongated, is rectangular or whose narrow sides are inclined with respect to the longitudinal edges of the platelet. Such an inclined direction makes it possible for acoustic waves which strike against this end edge of the platelet and run in the main wave propagation direction to be reflected back on this edge in a direction other than the main wave propagation direction. However, it is in general normal instead of this or in addition to this to fit a so-called sump on or to the substrate platelet in the region of a relevant end edge of the substrate platelet. The material of such a sump has the property of absorbing waves of the surface-wave arrangement which pass into the region of this sump, that is to say no longer allowing them to pass back into the structures which are fitted on the substrate surface.
From the documents GB-A-2,048,010, EP-0,057,598 and DE-B-2,738,192, it is known for the outer border of a spot of absorbent material which is fitted on the substrate surface to be given a specific shape, for example as a large-area triangle having a zig-zag edge or some other irregular shape. Another shape (DE-B-2,738,192, FIGS. 4, 5 and 6) is that having a sawtooth shape. If such a sawtooth-shaped edge faces the wave, total reflection effects are thus precluded (which occur in the case of the invention which is still to be described). A sawtooth shape along the rearward edge with respect thereto would be virtually insignificant, even if said edge were to satisfy the angle conditions for total reflection, since the wave is essentially already attenuated to virtually zero, anyway, at this location.
Such materials for acoustic absorption are plastics and the like which are crosslinked with solvents and whose evaporations continuously change the electrical characteristics of a surface-wave arrangement, even over a lengthy time. When the arrangement is in a closed housing, such a material is highly disadvantageous for high-quality arrangements.
Proposals have thus been made, specifically as cases for acoustic waves, to produce incisions, of arranged angle, in the arrangement in the material of the rearside of the substrate platelet (DE-B-2,450,831) or to cut angled notches into the edges of the substrate (DE-A-2,131,614). For a surface-wave arrangement having a piezoelectric layer on a non-piezoelectric substrate, JP-A-60-256214 proposes that the edge of this piezoelectric layer be allowed to run in a zig-zag. Apart from their small effectiveness, such measures are relatively costly.
A method which is very effective even without absorbent plastic and without incisions into the substrate material is specified in the document WO 89/02675, which has already been mentioned at the start. Said document describes a conversion structure which is a metallization of the substrate surface and comprises strip arrangements such as those which are also used for resonator arrangements and reflector arrangements. However, since these conversion structures are not a component of the actual surface-wave arrangement, these conversion structures are also located outside the region of the structures which determine the function of the surface-wave arrangement as a resonator or as a filter. Such a conversion structure has a dimensioning, which is described in greater detail in the document, serves for the purpose of, and results in an acoustic surface wave running in the surfaces of the substrate body being converted into a volume wave, that is to say being deflected out of the surface of the substrate body into its volume, and disappearing there in the mass body of the substrate wafer.
The reason for the known use of a sump, inclined ends of the substrate, of incisions into the substrate or of a conversion structure is to prevent surface waves which would otherwise be reflected in an undesirable manner on the edges of the substrate body occurring and causing interference when they pass back into the actual structures of the surface-wave arrangement, for example of a filter, in particular allowing undesired signals to occur in the locking region of such a filter. Inclined ends of the substrate platelet specifically cannot be used in the same way as the measure intended here, particularly when the surface-wave arrangement is a multi-track arrangement in the case of which surface waves run in a first track having one main wave propagation direction and in a second track having the same main wave propagation direction, but offset parallel, and, possibly, in further parallel tracks. The substrate edge which is directed in an inclined manner would then have a negative effect, specifically in that surface-wave components of the one track striking against this edge would be reflected into the other track. It should be noted that such multi-track arrangements can also have tracks which deliberately run in an inclined manner, which is, for example, a measure for electro-acoustic coupling of such parallel tracks. Filters of such a type are known from WO 90/03690, WO 90/03692 and WO 90/03691 and JP-A-57-208719. In FIG. 2 of the last-mentioned document, a triangular reflector coating is used for the purpose of converting the transducer which is adjacent to this coating into a transducer which is unidirectional irrespective of the frequency. In this case, consideration has also been given to total reflection, for which purpose, however, a material would first have to be found whose total reflection angle is greater than 45.degree.. The double 45.degree. reflection of the wave is, specifically, a precondition for achieving the unidirectional effect of the transducer. A further example of this document relates to the use of such triangular reflectors as means for coupling the waves of two adjacent tracks of the wave exploitation in one respective surface-wave arrangement. In this context, DE-A-3,811,989 should also be mentioned, which specifies metallization structures whose inclined-angle edges are used for such inclined directions of the waves in a convolver.